Hybrid manufacturing of absorbable Hydroxyapatite-Wollastonite/Mg3Zn interpenetrating phase composites for bone substitutes

Hydroxyapatite-Wollastonite/Magnesium alloy (HAp-Wol/Mg3Zn) Interpenetrating Phase Composites (IPCs) were developed by combining two different fabrication technologies, giving rise to a hybrid route for manufacturing innovative biomedical composites. A triply periodic minimal surface (TPMS) geometry was modelled, exploring two different volume fractions to control ceramic/metal ratio and interconnectivity. HAp-Wol structures were manufactured using additive manufacturing vat photopolymerization, specifically Digital Light Processing technology. The thermal debinding and sintering process of the printed HAp-Wol ceramic were optimized to allow a subsequent infiltration of the TPMS structures with a Mg3Zn alloy via investment casting, materialising the designed interpenetrating phase composites (IPCs). This approach enabled an effective infiltration of the alloy without the presence of major defects. The compressive strength of the HAp-Wol/Mg3Zn IPCs was significantly higher than that of the ceramic counterparts, increasing from 6.5 MPa to 162.5 MPa for the IPCs having the lowest ceramic ratio. The ceramic phase evidenced the formation of an apatite-like layer at their surface upon in vitro dissolution tests, evidencing bioactivity, being the calcium-rich silicate phase strongly responsible for this behaviour. IPCs revealed a higher dissolution rate than the ceramic counterpart. This study demonstrates the feasibility of this hybrid manufacturing route to fabricate HAp-Wol/Mg3Zn IPCs that can be tailored by design to meet the requirements of bone substitutes, namely the mechanical performance and absorption rate.